Lubricating base oils having the above properties are sometimes referred to as a API Group II base oils as defined in API Publication 1509: Engine Oil Licensing and Certification System, “Appendix E-API Base Oil Interchangeability Guidelines for Passenger Car Motor Oil and Diesel Engine Oils”. These properties are also mentioned in Oil & Gas Journal, Sep. 1, 1997, pages 63-70.
In the above referred to Oil & Gas Journal article various routes to Group-II base oils are described. All of the possible routes will involve at one point a hydrogenation of the aromatics and other unsaturated compounds to obtain a base oil having the desired saturates content. Such a hydrogenation will typically be performed by contacting the feed with hydrogen in the presence of a hydrogenation catalyst, typically a Group VIII metal supported catalyst.
U.S. Pat. No. 5,935,416 describes a process to prepare API Group II base oils, wherein the above to referred hydrogenation step is performed as a so-called cold (mild) hydrofinishing step at a temperature of between 230 and 300° C. The disclosed catalyst used in the hydrofinishing step is a catalyst containing Group VIB metals and a non-noble Group VIII metal.
WO-A-9802502 discloses a process to prepare API Group II base oils, wherein the hydrogenation process step is performed using a hydrogenation catalyst comprising a platinum/palladium alloy. The illustrated reaction temperature of the hydrogenation was 232° C.
GB-A-1381004 discloses a process to prepare a base oil by contacting a feed with a nickel-tungsten on alumina catalyst at temperature of between 360 and 410° C. and subsequently contacting the effluent of said step with a catalyst comprising a crystalline aluminosilicate faujasite and palladium at a temperature of between 230 and 370° C. According to this publication hydrogenation and desulphurisation takes place in the first step and conversion of polycyclic naphthenes and isomerisation of paraffins takes place in the second step. This second step cannot be considered to be a hydrogenation step in view of the conversion reactions taking place in this step as catalysed by the crystalline aluminosilicate faujasite.
GB-A-1333826 discloses a hydrogenation step in a process to prepare base oils. In Example 1 of this publication a three-stage hydrogenation process is illustrated using a nickel on Kieselguhr catalyst, wherein the reaction temperature was 221° C. in the first stage, 232° C. in the second stage and 260° C. in the third stage. The increased severity of the hydrogenation conditions was necessary, according to this publication, to improve the colour.
Applicants have found it almost impossible to prepare in a practical manner water-white Group II base oils, especially having a saturates content of more than 95 wt % and up to 99 wt %, and especially starting from the more difficult feeds containing higher sulphur and/or nitrogen levels using conventional methods. Either colored Group II base oils or water-white products having a saturates content of less than 90 wt % were prepared. With a practical manner is here meant a process having an acceptable capacity and/or having an acceptable reactor volume.
Water-white base oils are desirable because, for example, it ensures the absence of heavy polyaromatics and other harmful species which are detrimental to the product quality, such as for example the oxidation stability. The colour is suitably expressed by means of Colour Saybolt. Preferably the base oil has a Colour Saybolt of above 20 and more preferably above 25. The method of determining Colour Saybolt is described in ASTM D-156 method.